Moulding method and apparatus

ABSTRACT

A method and apparatus are provided for moulding an article by feeding molten plastics material into a metering cavity ( 16 ), feeding a predetermined quantity of the material from the metering cavity into a mould cavity via a transition passage ( 26 ) adjacent the mould cavity and urging the molten plastics material from the transition passage into the mould cavity with a working stroke of a packing piston ( 24 ), until the packing piston closes a port ( 28 ) defined between the transition passage and the mould cavity and a leading face ( 32 ) of the packing piston forms part of the peripheral wall of the mould. Less than ninety percent of the mould cavity is filled with the molten plastics material when the packing piston starts its working stroke and the packing piston starts its working stroke while molten plastics material is still being fed from the metering cavity.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of application Ser. No.11/258,011 filed Oct. 26, 2005.

FIELD OF THE INVENTION

This invention relates to a moulding method and apparatus that hasparticular application in moulding of plastics materials.

BACKGROUND TO THE INVENTION

In a conventional injection moulder, synthetic plastic is heated to amolten state and the molten plastics material is then forced at highpressure, through gates, sprues and runners to a mould cavity.

The rates at which the mould cavities can be filled are limited by therate at which the molten plastics material can flow through the narrowsprues, runners and gates. The molten plastics material has to be fed tothe mould cavity at high pressures, which are costly to apply and whichrequire substantial structural strength of the mould, feed equipment,etc. These disadvantages can be overcome by increasing the crosssections of the passages via which the molten plastics material is fedto the mould, which also allows molten plastics material with inclusionssuch as long fibres, particulate matter from recycled plastics, or thelike, to be used. With increased cross sections of the feed passages,lower feed pressures are required.

However, large cross sectional feeds to mould cavities have thedisadvantage that the port where the feed enters the mould, needs to beclosed off when the molten plastics material is allowed to freeze in themould. This can be achieved by accumulating the molten plastics materialneeded to fill the mould cavity, in a cylindrical holding chamberadjacent the mould and closing the port with a piston, the leading faceof which becomes part of the peripheral wall of the mould cavity, whenclosed. However, in apparatus of this type, the holding chamber is opento the mould cavity and molten plastics material flowing into theholding chamber contacts the part of the mould cavity immediatelyadjacent the feed port, where it starts to freeze before the pistonforces the molten plastics material into the chamber.

Further, the piston face in this type of mould apparatus is typicallyinternally cooled to cool with the rest of the mould wall, when themolten plastics material in the mould cavity is frozen. When the pistonis withdrawn to refill the holding chamber for a following mould cycle,the molten plastics material that flows into the holding chambercontacts the cold piston face and begins to freeze. The partial freezingof the molten plastics material in some areas, prior to the pistonfilling the mould, increases the viscosity of the molten plasticsmaterial and thus requires higher feed pressure to fill the mould cavityand is prone to leaving marks on the moulded products.

The disadvantages of filling a large cross sectioned holding chamberadjacent the mould cavity, before urging the molten plastics materialinto the mould cavity, are ameliorated to some extent in the inventiondisclosed in U.S. Pat. No. 6,464,910, to Smorgon et al. This patentdiscloses a moulding cycle in which molten plastics material is fed toan accumulator, from where it is fed to the mould cavity at lowpressure, via a large cross sectioned passage and wherein the feed portof the passage leading into the mould cavity is closed by a piston of avalve. The molten plastics material is thus not accumulated in a holdingcavity immediately adjacent the mould, where some of the molten plasticsmaterial may have frozen.

However, in the Smorgon et al process, molten plastics material is fedcontinually from an extruder to the accumulator and from the accumulatorto the mould cavity, so that there is no volumetric control over thequantity of molten plastics material fed to the mould cavity. Instead,the pressure within the mould cavity is measured. The result is that themould cavity is filled completely before the piston of the valve startsto close the feed to the mould. The valve has large cross sectionaldimensions and the advancement of the valve piston feeds a considerablevolume of additional molten plastics material into the mould, thuscausing over filling of the mould. Smorgon does not describe whathappens to the overflow of molten plastics material fed to the mould,but it is presumably received in an overflow reservoir and goes towaste.

Further, in the Smorgon et al process, the molten plastics material isfed continuously under low pressure from the accumulator to the mouldcavity until the mould cavity is full and the pressure in the mouldincreases. Only when this increase in pressure is detected, does thevalve piston start its movement to close the mould. It can thus safelybe assumed that the flow of the molten plastics material is momentarilyinterrupted before the valve piston movement starts. The interruption ofthe flow momentarily increases the residence time of the molten plasticsmaterial in the valve adjacent the mould and causes changes in therheology of the plastics material. The valve piston then forces thismaterial into the mould cavity, resulting in visible marks and/or localweakness within the product. Apart from the stagnation that occurs inthe valve, the melt front velocity of molten plastics material thatflows into the mould is also momentarily disrupted, which affects thephysical properties and appearance of the moulded product.

One object of the present invention is to provide an improved mouldingmethod and apparatus which allow molten plastics material to be feduninterruptedly to a mould cavity through a large cross sectionedpassage.

Another object of the present invention is to provide an improvedmoulding method and apparatus which limit wastage by limitingoverfilling of a mould cavity with molten plastics material which is feduninterruptedly to the cavity.

BRIEF DESCRIPTION OF THE INVENTION

According to one aspect of the present invention there is provided amethod of moulding an article, said method comprising:

-   -   feeding mouldable material into a metering cavity;    -   feeding the mouldable material from the metering cavity into a        mould cavity via a transition passage, the transition passage        being defined adjacent the mould cavity; and    -   urging some of the mouldable material from the transition        passage into the mould cavity with a working stroke of a packing        piston, until the packing piston closes a port defined between        the transition passage and the mould cavity and a leading face        of the packing piston forms part of the peripheral wall of the        mould;    -   wherein a predetermined quantity of the mouldable material is        fed from the metering cavity into the mould cavity.

Less than ninety percent of the mould cavity may be filled with themouldable material when the packing piston starts its working stroke.

The packing piston may start its working stroke while mouldable materialis still being fed from the metering cavity and the packing piston mayclose an inlet into the transition passage from the metering cavity,during said working stroke.

The mouldable material may be fed from the metering cavity by a workingstroke of a metering piston that is displaceable within the meteringcavity and the displacement of the metering piston within the meteringcavity may be monitored, to control the volume of mouldable materialthat is fed from the metering cavity.

The position of the packing piston relative to the port may be monitoredat the end of the working stroke of the packing piston to determine ifthe mould cavity has been filled with a required quantity of mouldablematerial, i.e. to determine if it has been over-filled or under-filled,and the distance that the metering piston travels during a subsequentworking stroke may be automatically adjusted to correct the quantity ofmouldable material that is fed to the mould cavity from the meteringcavity, i.e. to compensate for any over-filling or under-filling.

Instead, the mouldable material may be fed from the metering cavity tothe mould cavity by a working stroke of a reciprocating injectionmoulding screw that is displaceable within the metering cavity.

The temperature of the walls of the transition passage and/or of theface of the packing piston may be controlled.

The mouldable material may be fed from a single metering cavity tomultiple transition passages and from there to a single mould cavity orto multiple mould cavities. In the event that the mouldable material isurged into a single mould cavity from multiple transition passages, thepacking pistons of the transition passages may perform their workingstrokes at different times.

Mouldable material may instead be fed from multiple metering cavities toa single mould cavity.

The mouldable material may comprise, at least in part, of a moltenplastics material, or any other mouldable material.

According to another aspect of the present invention there is providedapparatus for moulding an article, said apparatus comprising:

-   -   a metering chamber and a displaceable member, together defining        a metering cavity, the volume of said metering cavity being        variable by reciprocal displacement of the displaceable member        in the metering cavity and said metering cavity being        connectable to a supply of mouldable material;    -   a packing chamber and a packing piston, together defining a        transition passage that is in flow communication with an inner        cavity of a mould via a port and that is in flow communication        with the metering cavity via a passage, the packing piston being        reciprocally displaceable within the transition passage between        a retracted position and a forward position, the packing piston        closing said port between the transition passage and the mould        cavity and a leading face of the packing piston forming part of        the peripheral wall of the mould cavity, when the packing piston        is in said forward position;    -   wherein the volume of the metering cavity is configured to be        varied by a predetermined amount, during the reciprocal        displacement of the displaceable member.

The packing piston may be configured to start a working stroke from itsretracted position to its forward position when less than ninety percentof the mould cavity is filled with mouldable material and may beconfigured to start its working stroke while mouldable material is stillbeing fed from the metering cavity.

The displaceable member may be a metering piston or an injectionmoulding screw.

The packing piston may include means for controlling the temperature ofits leading face and similarly, the packing chamber may include meansfor controlling the temperature of the transition passage.

The apparatus may define a metering cavity that is connected to multipletransition passages, which in turn may be in flow communication withmultiple mould cavities or with a single mould cavity.

Instead, the apparatus may define multiple metering cavities that areconnected via multiple transition passages to a single mould cavity.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, and to show how thesame may be carried into effect, reference will now be made, by way ofnon-limiting example, to the accompanying drawings in which:

FIG. 1 is a section illustrating moulding apparatus in accordance withthe present invention in a first operative condition; and

FIGS. 2 and 3 are views similar to that of FIG. 1 and illustrating, withFIG. 1, the operating cycle of the apparatus.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring to the drawings, moulding apparatus in accordance with theinvention is indicated generally by reference numeral 10. Some of thefeatures of the apparatus 10 are only visible in some of the drawings.

The apparatus 10 comprises a metering chamber 12 within which there is areciprocally displaceable member in the form of a metering piston 14, sothat a metering cavity 16 of variable volume is defined inside themetering chamber. The metering cavity 16 has an inlet opening 18 thatcan be connected to a supply of molten plastics material, e.g. a heatedpipe leading from an extruder or compounder, and has an outlet leadinginto a passage in the form of a melt flow path 20. The metering piston14 is upwardly and downwardly displaceable between a fully down positionshown in FIG. 1 and raised position shown in FIGS. 2 and 3 and is drivenby external means such as a double acting hydraulic cylinder (notshown). The position and displacement of the metering piston 14 ismonitored accurately and the height by which it is lifted to its raisedposition is accurately controlled, by using a digital potentiometer.

It would be appreciated by a person skilled in the art that the supplyof molten plastics material may include a method of plasticizing and/orcompounding the polymer and/or additives or reinforcing means asmentioned above and this can be achieved by using different types ofscrews, such as single or twin screws, continuous, intermittently or ofa stop/start type, etc.

The metering cavity 16 can be substituted by a conventionalreciprocating injection moulding screw which would provide a meteredquantity of molten plastics material to the mould cavity. However, aconventional moulding screw is optimised for plasticizing and not forstrokeability or measurement, whereas a metering piston 24 as describedabove, can have a stroke length to diameter ratio of up to 20:1, givingexceptional metering accuracy.

The apparatus further comprises a packing chamber 22 in the form of abarrel within which there is a reciprocally displaceable packing piston24, so that a cavity in the form of a transition passage 26 (as can bestbe seen in FIG. 3) is defined within the packing chamber. The packingchamber 22 defines an inlet 28 in its walling, from the melt flow path20 into the transition passage.

The transition passage 26 is defined immediately adjacent the mouldcavity (not shown) and is open to the mould cavity, so that a port 30 isdefined where the transition passage opens into the mould cavity. Themould defining the mould cavity has been omitted form the drawings as itis a cavity that can have an unlimited number of shapes and dimensionsand it is thus not readily capable of being illustrated. It will beclear to those skilled in the art that the mould cavity will, in use, beimmediately to the left of the transition passage 26, as illustrated.

The packing piston 24 is driven externally, e.g. by a double actinghydraulic piston, to be displaceable between a retracted position shownin FIG. 3 and a forward position shown in FIGS. 1 and 2. When thepacking piston 24 is in its forward position, it closes the port 30between the transition passage 26 and the mould cavity, and a leadingface 32 of the packing piston forms part of the peripheral wall of themould cavity. The packing piston 24 also closes the inlet 28, when inits forward position. When the packing piston 24 is retracted, the inlet28 and the port 30 are open and the inlet 28 is spaced a small distancefrom the piston's leading face 32.

Those skilled in the art would appreciate that the movements of themetering piston 14 and packing piston 24 can be controlled in a numberof ways, e.g. by using hydraulics with linear encoders and proportionalcontrol valves, hydraulically driving with electrically adjustablethreaded stops, recirculating ball screws operated by servo motors,linear motors, or the like.

In use, when the mould cavity is filled with molten plastics materialthat can freeze to form a moulded article or part, the apparatus is inthe condition shown in FIG. 1, with the packing piston 24 in its forwardposition and the metering piston in its fully down position.

While the molten plastics material in the mould cavity freezes, themetering cavity 16 is filled with molten plastics material that entersthe metering cavity via the inlet opening 18 and the metering piston 14gradually lifts to increase the volume of the metering cavity toaccommodate the inflow, until the apparatus 10 is in the condition shownin FIG. 2, in which the metering piston 14 is at a predetermined raisedposition, so that a predetermined metered volume can be displaced by themetering piston between its raised and fully down positions. Themetering cavity 16 thus receives molten plastics material even while thematerial in the mould cavity is still freezing, thus keeping the cycletime for the moulding process short.

Once the plastics material in the mould cavity has frozen, the mould isopened and the article ejected. The mould then re-closes and the mouldcavity is ready to be filled with molten plastics material to makeanother article in a next cycle of operation, the packing piston 24 iswithdrawn to its position shown in FIG. 3, to open the inlet 28 and theport 30.

The metering piston 14 is lowered a predetermined distance from itsraised position to its fully down position, so that the predeterminedmetered volume of molten plastics material is displaced by the meteringpiston, from the metering cavity 16, via the melt flow path 20 and theinlet 28, into the transition passage 26, from where it flowsuninterrupted through the port 30 into the mould cavity. The volume ofmolten plastics material fed from the metering cavity 16 during thisstroke of the metering piston 14, generally equals the volume requiredto make an optimised quality article in the mould cavity.

While the metering piston 14 moves downwardly, the molten plasticsmaterial flows continuously and uninterrupted from the metering cavity16 to the mould cavity, so that the residence time of the plasticsmaterial in the transition passage 26 is kept to a minimum.

While the metering piston 14 is still travelling downwardly, but isnearing its fully down position, the packing piston 24 starts a workingstroke from its retracted position, towards the port 30. When thepacking piston 24 reaches the inlet 28, it closes it. In the illustratedembodiment of the invention, the packing piston 24 first travels a shortdistance from its retracted position, before it reaches the edge of theinlet 28 and starts closing the inlet. This is not essential, but itallows the packing piston 24 to accelerate and start displacing moltenplastics material from the transition passage 26 and allows the meteringpiston 14 to reach its fully down position so that flow in the melt flowpath 20 stops, before the packing piston reaches the inlet 28. Theapparatus 10 is configured so that the metering piston 14 reaches itsbottom position just as the packing piston 24 is about to start closingthe inlet 28.

The metered volume displaced by the metering piston 14 generally equalsthe volume of the mould cavity, while the volume of the transitionpassage 26 is made as small as possible. However, the cross section ofthe transition passage 26 should preferably be large for reasonsprovided above and the length of the packing piston's stroke cannot besmaller than what is allowed by physical constraints such as thethickness of the mould wall, the diameter of the melt flow path 20, etc.The volume of the transition passage 26 is preferably about ten percentto forty nine percent of the metered volume and it follows that themould cavity is filled about fifty one percent to ninety percent withmolten plastics material that is fed by movement of the metering piston,before the last ten to forty nine percent of the mould cavity is filledby material fed by movement of the packing piston.

The overlapping reciprocal movements of the metering piston 14 and thepacking piston 24 causes the flow of molten plastics material from thetransition passage 26 into the mould, to be continuous and thusmaintains the melt front velocity as the material flows into the mouldcavity. The flow may be accelerated momentarily when both the pistons14,24 are moving, but this is generally inconsequential. What isimportant, is that the flow of molten plastics material does notstagnate in the transition passage 26 and that the residence time ofmolten material in the transition passage is kept to a minimum. Theuninterrupted flow of material during the process is especiallybeneficial when moulding materials that can be adversely affected bylong residence times during the injection cycle.

Keeping the residence time of plastics material in the transitionpassage 26 to a minimum, is advantageous since the face 32 of thepacking piston 24 and the walls of the transition passage 26 adjacentthe mould cavity may be cooler than the internal walls of the meteringcavity 16 and the melt flow path 20, owing to thermal losses that occurwhen the walls of the mould cavity (including the piston face 32) arecooled to form an article in the mould. In preferred embodiments of theinvention, the piston face 32 and/or the packing chamber 22 can becooled and/or heated separately and independently and can be kept attemperatures that differ from those of the cooled walls of the mouldcavity. Those skilled in the art would appreciate that all the apparatus10 would normally be heated and/or cooled, as desired, to keep theapparatus at temperatures at which the plastics material remains molten,but at which thermal degradation is kept to a minimum. However, theouter walls of the mould (which could include the piston face 32 inembodiments where cooling has been provided for the piston face) have tobe cooled to freeze molten plastics material in the mould cavity andthermal losses from parts of the apparatus immediately adjacent themould, are practically inevitable.

The stroke of the packing piston 24, which could be cooled internally,should preferably be kept as short as possible in order to limitdegradation and get the best surface finish. The ratio of the length tothe diameter of the packing piston 24 could be in the range of 7:1.

When the packing piston 24 reaches its forward position, its face 32forms part of the mould wall and the molten plastics material in themould can be allowed to freeze. The apparatus 10 is now again as shownin FIG. 1 and the cycle is complete. During the start of the mouldingcycle, the mould can be slightly open and can start to close during orafter the working stroke of the packing piston, similar to conventionalinjection compression moulding techniques, although in most cases, themould would be closed at the start of the injection cycle.

The position of the packing piston 24 relative to the port 30 ismonitored at the stage when it reaches its forward position, to monitorthe extent of filling of the mould cavity with the volume of moltenplastics material fed from the metering cavity. The stroke of themetering piston 14, i.e. the height to which it is raised in thefollowing operational cycle can be automatically adjusted to correctover or under filling of the mould cavity in relation to the stoppingposition of the packing piston in its forward stroke.

The inlet opening 18, melt flow path 20, inlet 28, transition passage 26and port 30 are all of cross sectional diameters that far exceed thediameters of the narrow sprues, runners and gates of conventionalinjection moulding apparatus. The size of the port 30 is at lest twicethat of the gate in conventional injection moulding equipment. As aresult, the melt velocity of the molten plastics material at the port 30is at least 10 times less than what is the case in an optimisedconventional injection moulding process. Further, the time required tofill the mould cavity is less than half that required to fill a mouldcavity in a conventional injection moulding process and because fillinghappens so much faster, the temperature of the front face 32 of thepacking piston 24 can be controlled in such a way as to get the mostoptimal surface finish and/or properties in the moulded part, whichcould lead to a decrease in total cycle time as well as a decrease inthe dimensional instability (warping) of moulded articles. The workingstroke of the packing piston 24 should preferably be kept as short aspossible in order to limit degradation and get the best surface finish.

An additional benefit of this low shear moulding process through a largegate is that thinner in-mould decorative skins and other layers can beused that would not be pierced as easily as what can be observed inconventional injection moulding processes. The other in-mould layers caninclude fibre mats, PET, metal, textiles, or the like.

The moulding of long glass fibres with an average length in excess of 4mm in a polyolefin matrix can be achieved by using the method of thepresent invention, due to the uninterrupted, minimal restrictivepassages or gates that the composite has to move through before enteringthe mould.

In conventional injection moulding there is a direct relation betweenthe weight of material fed to a mould cavity (i.e. the shot weight) andthe clamping force required to keep the mould closed. The presentinvention allows the shot volume to be increased in relation to theclamping force required and therefore allows a smaller extruder to beused with a 100% duty cycle to supply two moulds with molten plasticsmaterial, whether virgin material or recycled material.

Low melt fracture index (MFI) materials can be moulded by the method ofthe present invention with a reduced likelihood of getting under-fillingof the mould cavity (referred to as “short shots”). Further, since therate at which material is being fed into the mould cavity is beingcontrolled, dilitant materials can be moulded using the presentinvention. Temperature sensitive materials such as wood fibre or fillerscan also be moulded to a ⅔ dimensional geometry due to the very lowshearing and minimal increased heat.

The present invention can be implemented in many differentconfigurations with single, multiple and/or branched product streams. Inparticular, a single metering piston 14 or plasticizing screw can beused to supply metered quantities of molten plastics material tomultiple transition passages 26, opening into a single mould cavity orinto multiple mould cavities. Further, the packing pistons 24 can beconfigured and controlled to perform their working strokes at differenttimes. This will allow the formation of weld lines that are formed whenmaterial fed from different transition passages 26 meet in the mouldcavity, to be controlled and/or predicted. The packing pistons 24 can becascaded to allow the mould cavity to be filled rapidly with reducedpressure.

In an alternative embodiment, different molten materials can be suppliedfrom different metering cavities and can be fed into the mould cavity inpredetermined locations and/or in a predetermined sequence. This allowsthe use of different materials in different parts or moulded articles,the use of different materials in a laminated configuration within themoulded product e.g. to provide a good surface finish with a differentmaterial used underneath a surface layer, etc. In these embodiments ofthe invention, the exact volume of each material that is fed to themould cavity and the rate of flow must be controlled accurately, asallowed by the present invention, since it affects the properties of themoulded articles. There is thus no limit to the number of differentmaterials that could be fed to the mould cavity, other than limitationsof the dimensions of the metering chamber, the moulding unit, the lengthof the transition passage and hot runner, and the like.

The present invention allows for the productivity of plasticizing unitsthat supply molten plastic material to moulding units, to be optimised,by using a single plasticizing unit to supply molten material todifferent moulding units, each with its own apparatus 10, moulds,clamping apparatus etc. The molten material from the plasticizing unitcan be fed continually to all the metering cavities 16 of the differentmoulding units, instead of feeding material intermittently as is done inconventional injection moulding. This allows for the use of smallerplasticizing units and/or lower energy consumption in the plasticizingunits.

The invention claimed is:
 1. A method of moulding an article, saidmethod comprising: feeding mouldable material into a metering cavity;feeding the mouldable material from the metering cavity into a mouldcavity via a transition passage, the transition passage being definedadjacent the mould cavity; and urging some of the mouldable materialfrom the transition passage into the mould cavity with a working strokeof a packing piston, until the packing piston closes a port definedbetween the transition passage and the mould cavity and a leading faceof the packing piston forms part of the peripheral wall of the mould;characterised in that a known, predetermined, metered volume of themouldable material is fed from the metering cavity by a working strokeof a metering piston displaceable member that is displaceable within themetering cavity, into the mould cavity and the displacement of themetering piston within the metering cavity is monitored to control thevolume of mouldable material that is fed from the metering cavity,further characterised in that the packing piston starts its workingstroke while mouldable material is still being fed from the meteringcavity, such that an uninterrupted flow of mouldable material in thetransition passage is maintained.
 2. A method as claimed in claim 1,characterised in that less than ninety percent of the mould cavity isfilled with the mouldable material when the packing piston starts itsworking stroke.
 3. A method as claimed in claim 1, characterised in thatthe packing piston closes an inlet into the transition passage from themetering cavity, during said working stroke.
 4. A method as claimed inclaim 1, characterised in that the position of the packing pistonrelative to the port is monitored at the end of the working stroke ofthe packing piston to determine if the mould cavity has been filled witha required quantity of mouldable material, and the distance that themetering piston displaceable member travels during a subsequent workingstroke is automatically adjusted to correct the quantity of mouldablematerial that is fed to the mould cavity from the metering cavity.
 5. Amethod as claimed in claim 1, characterised in that the temperature ofthe walls of the transition passage is controlled.
 6. A method asclaimed in claim 1, characterised in that the temperature of the face ofthe packing piston is controlled.
 7. A method as claimed in claim 1,characterised in that the mouldable material is fed from a singlemetering cavity to multiple transition passages.
 8. A method as claimedin claim 7, characterised in that the mouldable material is fed from themultiple transition passages to a single mould cavity.
 9. A method asclaimed in claim 7, characterised in that the mouldable material is fedfrom the multiple transition passages to multiple mould cavities.
 10. Amethod as claimed in claim 1, characterised in that the mouldablematerial is urged into a single mould cavity from multiple transitionpassages and the packing pistons of the transition passages performtheir working strokes at different times.
 11. A method as claimed inclaim 1, characterised in that mouldable material is fed from multiplemetering cavities to a single mould cavity.
 12. A method as claimed inclaim 1, characterised in that the mouldable material comprises, atleast in part, of a molten plastics material.
 13. Apparatus for mouldingan article, said apparatus comprising: a metering chamber and adisplaceable member, together defining a metering cavity, the volume ofsaid metering cavity being variable by reciprocal displacement of thedisplaceable member in the metering cavity and said metering cavitybeing connectable to a supply of mouldable material; a packing chamberand a packing piston, together defining a transition passage that is inflow communication with an inner cavity of a mould via a port and thatis in flow communication with the metering cavity via a passage, thepacking piston being reciprocally displaceable within the transitionpassage between a retracted position and a forward position, the packingpiston closing said port between the transition passage and the mouldcavity and a leading face of the packing piston forming part of theperipheral wall of the mould cavity, when the packing piston is in saidforward position; characterised in that the displaceable member is ametering piston and the volume of the metering cavity is configured tobe varied by a known, predetermined amount, during the reciprocaldisplacement of the metering piston displaceable member, furthercharacterised in that the packing piston is configured to start itsworking stroke while mouldable material is still being fed from themetering cavity, such that an uninterrupted flow of mouldable materialin the transition passage is maintained.
 14. Apparatus as claimed inclaim 13, characterised in that the packing piston is configured tostart a working stroke from its retracted position to its forwardposition when less than ninety percent of the mould cavity is filledwith mouldable material.
 15. Apparatus as claimed in claim 13,characterised in that the packing piston includes means for controllingthe temperature of its leading face.
 16. Apparatus as claimed in claim13, characterised in that the packing chamber includes means forcontrolling the temperature of the transition passage.
 17. Apparatus asclaimed in claim 13, characterised in that it defines a metering cavitythat is connected to multiple transition passages.
 18. Apparatus asclaimed in claim 17, characterised in that the multiple transitionpassages are in flow communication with multiple mould cavities. 19.Apparatus as claimed in claim 17, characterised in that the multipletransition passages are in flow communication with a single mouldcavity.
 20. Apparatus as claimed in claim 13, characterised in that itdefines multiple metering cavities that are connected via multipletransition passages to a single mould cavity.
 21. A method as claimed inclaim 1, characterised in that the displaceable member is a meteringpiston.
 22. A method as claimed in claim 1, characterised in that thedisplaceable member is an injection moulding screw.
 23. A method asclaimed in claim 13, characterised in that the displaceable member is ametering piston.
 24. A method as claimed in claim 13, characterised inthat the displaceable member is an injection moulding screw. 25.Apparatus for moulding an article, said apparatus comprising:displaceable means for varying a volume of a metering cavity by, saidmetering cavity being connectable to a supply of mouldable material; apacking chamber and a packing piston, together defining a transitionpassage that is in flow communication with an inner cavity of a mouldvia a port and that is in flow communication with the metering cavityvia a passage, the packing piston being reciprocally displaceable withinthe transition passage between a retracted position and a forwardposition, the packing piston closing said port between the transitionpassage and the mould cavity and a leading face of the packing pistonforming part of the peripheral wall of the mould cavity, when thepacking piston is in said forward position; characterised in that thevolume of the metering cavity is configured to be varied by a known,predetermined amount, during the displacement of the displaceable means,and further characterised in that the packing piston is configured tostart its working stroke while mouldable material is still being fedfrom the metering cavity, such that an uninterrupted flow of mouldablematerial in the transition passage is maintained.